The present invention relates to a novel type of fin-stabilized missiles which can be guided in their respective trajectories towards a predetermined target. Guidable missiles here signify guidable artillery shells, rockets or projectiles. These are assumed here to be of the general type which are preferably fired without rotation, or at a low inherent rotation about their longitudinal axis, and which, for stabilizing them in their trajectory towards the target, are assumed to be provided with stabilizing fins which are arranged at the rear end and are initially retracted until the missile has completely exited the launch arrangement from which it has been fired, and can then be deployed once it has left the launch arrangement completely. To guide the missiles in pitch and yaw in their trajectories towards their intended targets, they are also assumed to be provided with control members arranged for this purpose preferably at their front end.
In many cases it is desirable, as it is in the present invention, to be able to guide missiles (for example shells, rockets or projectiles) towards a defined target while the missiles are in their trajectory. This can be done, for example, by guiding them in pitch and yaw by means of control members arranged at the front end of the missile, and these members can consist for example of canard fins, jet nozzles, etc.
Airborne missiles can be rotation-stabilized in their trajectory or stabilized in another way, for example by means of fins. Rotation-stabilized missiles have steady trajectories and they can be made mechanically simple since the launch arrangement as a rule is responsible for ensuring that the missile acquires the necessary initial rotation. However, the high rotational velocity has at least hitherto made it impossible to provide this type of missile with a well-functioning guidance system. When work is undertaken today to develop effective guidable missiles, one has therefore concentrated efforts on missiles which do not rotate at all, or rotate only slowly, about their own longitudinal axis and which are aerodynamically stabilized by means of fins arranged in their rear part.
In addition to stabilizing the missile flight, the stabilizing fins, in a fin-stabilized nonrotating missile, or in a missile rotating only slowly, can additionally give rise to an active lifting force which acts on the missile and can be used to increase its range of fire.
A current trend in the development of artillery technology is towards new long-range artillery missiles guided in their final phase, and interest has increased in different types of fin-stabilized shells intended for firing in conventional guns and howitzers. To make it possible to launch fin-stabilized shells with a low inherent rotation directly from grooved barrels, the shells need to be provided with a drive band as their only direct contact with the grooving of the barrel. The same gun or howitzer can thus be used, without special intermediate measures, to successively fire essentially nonrotating shells provided with drive bands and with stabilizing fins, which can be deployed in trajectory, and entirely conventional rotation-stabilized shells.
In controlling the trajectory of fin-stabilized missiles such as shells, rockets and projectiles, it is necessary to know and be able to control the roll position of the missile. This in order to be able to control the missile in pitch and yaw. This control is achieved preferably with special control elements, for example in the form of movable nose fins, called canard fins, or jet nozzles. However, the roll control moment which such control elements in the front part of the missile give rise to can in many cases be counteracted or completely eliminated by the guide fins in the rear part of the missile, unless special measures are taken. This is due to the fact that the vortices caused by the control moment from the rudder or other control activity impact the fins and this in turn gives rise to a counteracting moment.
A way of solving this problem which has already been tested to an at least limited extent is to let the part of the missile in which the fins are secured constitute a unit which can rotate freely in relation to the rest of the missile about an axis concentric with the longitudinal axis of the missile. In this way, the effect of the control moment on the fins cannot be transferred to the front part of the missile, as a result of which the missile is made easier to control.
From a purely practical point of view, it might be considered very easy to design a freely rotating bearing between the main part of the missile and a fin unit connected to the latter, but in reality this is not such a straightforward matterxe2x80x94indeed it is extremely complicatedxe2x80x94since all the parts of the bearing have to be dimensioned in a way which takes into account the stresses in the form of high acceleration and deceleration which these parts have to tolerate both during ramming and during launch, and the maximum forces which occur in these cases are also effected in different directions.
The basic principle of the freely rotating fin unit has therefore to be regarded as already known at least in terms of its main features. The present invention therefore relates more specifically to a missile provided with a specially designed freely rotating fin unit. The invention is also in the first instance intended to be applied to a fin-stabilized artillery shell, but it can also apply to any other fin-stabilized and slowly rotating missile of the abovementioned general type. The particular characteristic feature of the fin-stabilized missile according to the invention is thus the design of the bearing for the freely rotating fin unit. This bearing has now been designed to tolerate the acceleration and deceleration forces during ramming of the shell and then the acceleration forces during firing of the shell.
The fin stabilizing unit forming part of the shell according to the invention thus comprises a specific body part in which the fins are secured and relative to which the fins can be retracted, and this body part can in turn rotate freely relative to the rest of the shell about a bearing which is concentric to the longitudinal axis of the shell. This bearing in turn comprises a ball bearing or roller bearing in a single bearing position with the greatest possible bearing diameter but with a very short length in the direction of flight of the missile, compared to said diameter, and this bearing position is additionally preferably arranged as close as possible to the dividing plane, running transverse to the longitudinal direction of the missile, between the rest of the missile and the fin stabilizing unit which rotates freely relative to the latter. The bearing which characterizes the invention moreover comprises specially designed pairs of interacting contact surfaces in both the main part of the shell and in the body part, arranged peripherally with respect to the freely rotating fin unit and activated in the axial direction upon maximum acceleration and deceleration stresses. In the preferred embodiment of the invention, these contact surfaces are designed in such a way that the acceleration and deceleration contact surfaces belonging to either the freely rotating body part or the main part of the missile are oriented in opposite directions, which means that the contact surfaces in the body part are directed towards each other while those in the main part of the missile are directed away from each other. In a development of the invention, there is also a specifically designed spring system whose task it is to take up within certain limits those forces which act in the longitudinal direction of the shell between the rest of the missile and the body part of the fin unit and which act on these parts to move them away from each other. This spring system, which acts between one of the parts and one of the drive rings of the ball bearing, has the task of allowing the parts to rotate freely relative to each other even when they are stressed away from each other by a limited force, as will be the case when the missile is flying through the air with the fins deployed. At the same time the spring has a safety function in that it is intended to ensure that the abovementioned contact surfaces engage with each other before there is any risk of exceeding the maximum bearing load which the ball bearing tolerates. As soon as said maximum bearing load approaches, the counter effect of the spring will have been exceeded and the parts will have been fixed relative to each other by means of the contact surfaces having engaged with each other and the free mutual rotation having ceased. As soon as the excessive loading has ceased, the spring will then ensure that the parts return to their original positions and the free mutual rotation again becomes possible.
The invention also includes a specific development in which the points of attachment of the fins consist of an axially displaceable body part which from a first retracted position inside the rear end of the missile body in front of its usual rear plane can be pushed out to a second deployed position where the fins and their points of attachment are situated behind said rear plane and where the fins are free to unfold and where this body part at least in its pushed-out position can rotate freely relative to the rest of the missile. Said body part can be designed as a cylinder which in the original position is thus inserted in a cylindrical cavity in the rear part of the missile. The detailed design of the body part can then vary depending on which fin type is chosen. With fins of the wrap-around type or folding-fin type, which are arranged along the outer periphery of the body part and are initially folded in towards the latter, the body part can provide space for a base-bleed unit, while in other types of fins, for example those which in the retracted position are folded into axial tracks in the body part about axles transverse to the longitudinal axis, the base-bleed unit has to be divided up into a number of smaller parts, which in turn will mean that there is less space available for the base-bleed powder. With the body part inserted into the rear part of the missile, there are less stresses, when the missile is a shell, in particular on the bearing during ramming in the barrel of the artillery piece since the drive band of the shell can then be arranged on that part of the missile in which the body part is inserted in the original position.
To ensure that the system with an axially displaceable body part can at the same time give a freely rotatable fin part, the body part must comprise a first body section and a second body section, where the first body section is axially displaceable, but not rotatably connected to the rest of the missile, while the second body section is displaceable together with the first one and freely rotatable relative to it. When the body part is displaced between its two positions, these two sections are thus displaced axially to a position where the second body section lies completely outside the original rear plane of the missile and in this position the displacement of the first body section is locked for example by means of an abutment flange or other type of deformation lock between the parts.
To activate the pushing-out of the fin-supporting body part from its position inside the rear end of the missile to its extended position, different methods can be used, for example in the form of expanding pyrotechnic gases. In a method which is particularly well suited to artillery shells, during the actual launch some of the powder gases from the propellant charge of the firing equipment are introduced via a narrow channel into a chamber between the push-out body part and the rest of the missile, and after the missile has left the barrel and the powder gas pressure behind the missile has ceased, the expansion of these powder gases is used to drive the body part out to its outer position. The same method can also be used to remove a protective casing which during launch protects an axially immovable fin unit and which has to be removed before the fins can be deployed. This method, which has the advantages that it provides an extremely rapid reaction associated completely with the passage of the missile from the barrel muzzle, and that it is entirely without any need for extra components, is also described in more detail in connection with the examples below.